How do you declare what your "aniversary" date is?

I've been dating a guy for about 8 months now. I'm not entirely sure, because I dont remember exactly when our first date was, and neither does he. We are a little old to be celebrating monthly-versaries, so its never really been a problem. But one year is steadily approaching. Should I just make up a day and surprise him on it, and act aghast when he didnt know it was our "aniversary"? Does anyone else not really know when they started dating someone? LOL?